The present invention relates to an image tube device, and more specifically relates to an image tube device which converts incident electromagnetic waves into electrons and outputs an image on the basis of the electrons thus generated.
There have been developed various devices for converting electromagnetic waves such as X-rays transmitted from an object into electrons, and for outputting an observable image of the object on the basis of the electrons thus generated. FIG. 3 shows, as an example of such devices, an X-ray image magnification observing device.
The observing device 10 irradiates an object 12 with X-rays emitted from an X-ray source 11. X-rays transmitted from the object 12 are incident on a window 13 and then imaged by an X-ray magnification imaging means 14. A photocathode 15 is provided at the imaging position of the imaging means 14, and converts the X-rays into electrons. The photocathode 15 is formed on a supporting substrate 15a which is thin enough to transmit X-rays. The photoelectrons thus generated are accelerated by means of an acceleration electrode 16 along a direction generally identical to the X-ray incident direction, focused by means of an electromagnetic focusing coil 17, and are finally incident on a microchannel plate (hereinafter abbreviated as MCP) 18 provided on the electron traveling direction. The electrons are multiplied by the MCP 18, and are incident on a phosphor screen 19, where they are converted into a visible light image. By picking up the visible light image by, e.g., TV camera 20, an X-ray magnified image of the object 12 become observable.
In the conventional image observing devices of the above kind, a MCP and a phosphor screen are provided on the direction along which photoelectrons generated by a photocathode are accelerated. Since it is difficult to convert all the incident X-rays into electrons, it is sometimes the case that part of the incident X-rays just pass through the photocathode without being converted into electrons. Particularly as in the above case in which the photocathode is formed on the thin substrate, a larger amount of X-rays are transmitted. Since transmitted X-rays are not influenced by the focusing electromagnetic coil, they travel straight and are incident on the MCP and phosphor screen. As a result, the transmitted X-rays contribute to the output of the phosphor screen as background noise.
It may be conceivable that a shield member is provided on the traveling path of the X-rays. However, since the spread, in the plane perpendicular to its traveling direction, of a photoelectron beam emitted from one point of the photocathode is small, the photoelectron beam itself may strike the shield member, and therefore the incident X-ray corresponding to this particular photoelectron beam will not contribute to an output visible light image. Thus, the provision of such a shield member does not solve the above problem.